All units and cities have 10 health.
Although this is not 100% gaurenteed I think we have established this from the footage that exists.
So why is Japan's ability not like Civ-Rev's auto-heal ability.
Well its simple, as someone else mentioned, health and strength are seperated, and when health is 0 your unit dies.
Normally when your health is lower your "damage" (that your strength dictates) is also lower. Damage to an enemies health is decided by your health and your strength.
However in Japan's case, Health never factors into damage or "strength". (Note: Strength is never shown to decrease when low on health, this is why I invented the term damage)
This means that in melee they have the advantage, I will explain.
Assume a longswordsman deals 6 damage without damage modifiers. This means two full strength melee attacks will be needed to kill a 10 Health Unit.
(The actual damage a unit does depends on your strength, your health (accept in Japans case) and your enemies strength.)
The second melee attack is where things go different for Japan, a low health longswordsman should deal less than 6 damage, but Japan will still deal the full 6 damage, so to kill his longsword of 10 HP you take a total of 12 damage (which could be a lost unit on your side too). Ignoring all other multipliers.
Where as with another civ, the damaged sword would do say 3 damage in the second melee (a guess) and this means definitely no loss on your side to kill the enemy sword.
The way to get around this advantage (which isn't massively overpowered anyway) is to soften the target with ranged if you can't simply kill it with ranged, as now you only have to hit it once against its full strength retaliation to kill it because of its low health. You defintely won't lose a unit attacking but you will need to sit around and heal more HP than if you had ranged and melee'd a different Civ.
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Its not "over-powered" by a long way, however it is a decent ability and will make them great melee fighters. (Their low health ranged units will also do full damage which is cool

, but with no ranged retaliation its not that useful. Accept in making a counter attack.)
Their are more powerful military abilities such as Russia's "Double Horse, Double Iron, Double Uranium & +1 Prod on Strategic Resource Tiles."
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Also how "powerful" Japans ability is depends on the ratio of Health & Strength. We can safely assume it is less than 1:1 because if X reduction in health lead to an equal X reduction in Strength then the sepearation of Health & Strength is pointless. So depending on what the ratio is will depend how strong the ability is. If its close to 1:1 say 0.75:1 then the ability will be quite strong, where as if the ability is 0.25:1 the ability isn't that great.
It should be fairly easy to work out if you know what an enemys health & damage is before an attack, and then what their health & damage is afterwards. I will edit in the ratio if I figure it out.
Edit : Unfortunately the only footage I have found of the same unit surving two attack two units by melee before dying is on the "closed E3 Demo" footage and its too blurry to make out, but I can just about make out the fact that the Rifleman had 10 hp damage inflicted before dying so yeah I'm pretty certain 10hp is the health on everything.